Known switched-mode power supplies are made up of a primary side (AC input) and a secondary side (DC output), which is galvanically separated from the primary side.
Switched-mode power supplies have a control loop on the secondary side. The control loop ascertains the electric output voltage and the electric output current and compares these with the respective setpoint values. A signal resulting from this comparison controls an optocoupler. Said optocoupler transmits the signal to the primary side in that it draws or modifies a primary regulating current as a function of the secondary signal.
Additionally, in switched-mode power supplies with higher power output, usually the temperature is detected on the secondary side via an NTC thermistor so that in the event of an increased temperature (e.g., because of an increased ambient temperature or because the active cooling (ventilator) has failed), the power output is reduced by reducing the setpoint values (so-called “derating”).
Integrated switching controllers, which can be used on the primary side, often offer a function that switches off the switched-mode power supply completely in the event of an abnormal overtemperature (so-called latch). This can also be done via a bimetal switch. A complete switch-off is often not desirable, however, and moreover bimetal switches are usually expensive. WO 2004/027963 A1 discloses a switched-mode power supply, in which a control system is provided, in which a thermal model is implemented, with the aid of which it is possible to calculate or estimate the temperature of at least one component, the thermal model being provided with at least one load-dependent current value as electric variable. The control system is designed to output at least one limiting signal as a function of calculated or estimated temperature values when reaching specifiable limiting values or a function of multiple limiting values, which is usable in the sense of an intervention for temperature reduction. For this purpose, it is possible to supply the control system with a limiting signal, which acts on the control circuit in the sense of a temperature reduction and thereby a power output reduction.